The present invention relates to a system and method for carrying out a non-contact burn-in test on a semiconductor wafer.
Recently, the annual production of semiconductor devices has been rocketing year after year. Generally speaking, the greater the number of devices produced per unit time, the greater the number of devices with infant mortality to be screened out therefrom by an accelerated life test called xe2x80x9cburn-inxe2x80x9d, for example. As is well known in the art, a burn-in test is carried out on semiconductor devices by subjecting the devices to an elevated temperature under an electrical power stress. Some of the devices that failed to withstand the stress are screened out as NO-GOs, while the other devices that could endure the stress successfully are shipped as GOs, or good products. Over the past few years, however, the time afforded to develop new semiconductor devices has been more and more limited. So the burn-in test should also be finished in a shorter amount of time. In addition, a wafer test system for use in such a burn-in test also has to have its size further reduced, since the devices under test have been downsized almost day after day.
The burn-in test has normally been carried out by applying a stress voltage onto semiconductor devices on a wafer with probe pins brought into contact with the devices under test.
FIG. 14 illustrates how the burn-in test is carried out on a semiconductor wafer 301 including a great number of semiconductor devices thereon using a known wafer test system. As shown in FIG. 14, the wafer 301, supported on a substrate plate 302, is brought into contact with probe pins extending from a probe card 303, and then supplied with a signal delivered from a tester 304 through the pins of the card 303.
Next, it will be described how the wafer test system operates. In the example illustrated in FIG. 14, the plate 302 is grounded at a potential level of 0 V. The wafer 301 is in electrical contact with the plate 302, and each of the numerous devices on the wafer 301 also has its substrate potential fixed at 0 V. In such a state, the tester 304 outputs a signal to devices under test on the wafer 301 by way of the pins of the card 303. The devices under test, which are in contact with the pins of the card 303, start to operate in response to the signal supplied from the tester 304. As a result, a voltage is applied onto the gate electrode of each of those devices (i.e., transistors). That is to say, a voltage stress is generated between the gate electrode of the transistor and the substrate thereof. In this manner, the devices on the wafer 301 are subjected to the burn-in.
However, if test terminals provided for semiconductor devices on a wafer are of a different type from those provided for devices on another wafer, then the known wafer test system should prepare two mutually different types of probe cards for these two wafers.
To avoid such an undesirable situation, the present inventor performed a non-contact burn-in test on semiconductor devices on a semiconductor wafer without using any probe pins. In this burn-in test, each of the devices under test on the wafer was exposed to a direct-current (DC) electric field so that a voltage was applied onto the gate oxide film of the devices. Hereinafter, with reference to FIG. 10, I will briefly describe the burn-in test I conducted before describing the summary of my invention. FIG. 10 illustrates a semiconductor wafer test system that I used for the burn-in test. First, the respective elements of the system will be described.
As shown in FIG. 10, a semiconductor wafer 501, including a great number of semiconductor devices under the burn-in test, is supported on a substrate plate 502. The burn-in test is carried out by applying a predetermined voltage from a DC power supply 504 to a conductive plate 500 and by exposing the devices under test on the wafer 501 to an electric field S500 that has been created from the conductive plate 500. The electric field S500 created from the conductive plate 500 has an intensity proportional to the voltage applied from the power supply 504. As a result, a current I501 flows from the plate 502 into the ground.
FIG. 11 illustrates one of the devices under the burn-in test on the wafer 501 to a larger scale. First, the respective elements of the device will be described. As shown in FIG. 11, the semiconductor device (i.e., an MOS transistor in this case) to be exposed to the electric field S500 created from the conductive plate 500 has been electrically isolated from adjacent devices by isolation regions 501e and 501f. The device includes gate electrode 501a, gate oxide film 501b, source/drain regions 501c and 501d and p-well 501g. That is to say, part of the wafer 501 for this device includes the source/drain regions 501c and 501d, p-well 501g and substrate portion 501h. 
As also shown in FIG. 11, the wafer 501 is supported on the substrate plate 502. The device is exposed to the electric field S500 that has been created from the conductive plate 500 by applying a voltage from the DC power supply 504 to the conductive plate 500. A parallel plate capacitor is formed between the conductive plate 500 and gate electrode 501a and another parallel plate capacitor is formed between the gate electrode 501a and p-well 501g. A leakage resistor 512 exists between the gate electrode 501a and the ground and a current I501 flows from the substrate plate 502 into the ground. In FIG. 11, only one n-channel MOS transistor is illustrated as one of the great many devices on the wafer 501 for the sake of simplicity. Accordingly, the source/drain regions 501c and 501d have been doped with an n-type dopant, while the p-well 501g and substrate portion 501h are of p-type.
The substrate portion 501h is in electrical contact with the grounded substrate plate 502 and is fixed at 0 V. The p-well 501g is in contact with the substrate portion 501h and these regions 501g and 501h are both of p-type. So the p-well 501g is also fixed at 0 V.
When a voltage V0 (V) is applied to the conductive plate 500, the electric field S500 is created, thereby polarizing the gate electrode 501a and producing a voltage Va0 (V) at the gate electrode 501a. As a result, an electric field stress Ea (V/m) corresponding to the voltage Va0 (V) is placed on the gate oxide film 501b. Hereinafter, this stress will be analyzed quantitatively.
Suppose the area of the gate electrode 501a is Sa (m2), the distance between the conductive plate 500 and gate electrode 501a is d1 (m), the thickness of the gate oxide film 501b is d2 (m), the permeability between the conductive plate 500 and gate electrode 501a is xcex51 (C/(Vxc2x7m)) and the permeability of the gate oxide film 501b is xcex52 (C/(Vxc2x7m)). To simplify the computation, one parallel plate capacitor 510 is supposed to be formed between the conductive plate 500 and gate electrode 501a and another parallel plate capacitor 511 is supposed to be formed between the gate electrode 501a and p-well 501g as schematically illustrated in FIG. 12.
As also shown in FIG. 12, a voltage is applied from the DC power supply 504 to the conductive plate 500, thereby creating the electric field to which the device under test is exposed. A leakage resistor 512 exists between the gate electrode 501a and the ground.
Suppose no current flows through the resistor 512 for a while after the voltage V0 (V) has been applied to the conductive plate 500. Then, a quantity Q0 (C) of charge stored on the parallel plate capacitor 510 is given by the following Equation (1):
Q0=xcex51xc2x7S/d1xc3x97(V0xe2x88x92Va0)xe2x80x83xe2x80x83(1)
where Va0 (V) is the voltage induced at the gate electrode 501a. 
The charge quantity Q0 can also be obtained by the following Equation (2) using the quantity of charge stored on the parallel plate capacitor 511:
Q0=xcex52xc2x7S/d2xc3x97Va0xe2x80x83xe2x80x83(2)
Combining these Equations (1) and (2) together, the voltage Va0 (V) induced at the gate electrode 501a is given by the following Equation (3):
Va0=xcex51xc2x7d2/(xcex52xc2x7d1+xcex51xc2x7d2)xc3x97V0xe2x80x83xe2x80x83(3)
Accordingly, the electric field stress Ea0 (V/m) given by the following Equation (4):
Ea0=Va0/d2=xcex51/(xcex52xc2x7d1+xcex51xc2x7d2)xc3x97V0xe2x80x83xe2x80x83(4)
is placed on the gate oxide film 501b. Also, the intensity E0 (V/m) of the electric field S500 is given by the following Equation (5):
E0=xcex52/xcex51xc3x97Ea0xe2x80x83xe2x80x83(5)
It should be noted that the gate electrode 501a is grounded weakly due to the existence of metal interconnects and leakage current components. Accordingly, the induced charges gradually disappear with time. So if a DC voltage is applied to the conductive plate 500, then the electric field stress with the intensity Ea0 can be placed on the gate oxide film 501b for just a short period of time. To avoid this unwanted situation, if the electric field to be placed on the gate oxide film 501b to carry out the burn-in test is represented by E1 (V/m), then an electric field intenser than E1 (V/m) should be placed on the gate oxide film 501b initially.
Hereinafter, it will be described how this semiconductor wafer test system operates. First, the voltage V0 (V) to be applied to the conductive plate 500 will be considered.
In general, dielectric breakdown should occur even in a gate oxide film 501b with no defects if the gate oxide film 501b were exposed to an excessively high electric field. Accordingly, the voltage Va0 induced at the gate electrode 501a should be set to:
Va0=d2xc2x7E10xe2x80x83xe2x80x83(6)
where E10 (V/m) is a critical electric field with an intensity at and under which no dielectric breakdown occurs.
Combining the Equations (3) and (6) together, the critical electric field E10 (V/m) will be placed on the gate oxide film 501b initially if the voltage V0 given by
V0=(xcex52xc2x7d1+xcex51xc2x7d2)/xcex51xc2x7d2xc3x97d2xc2x7E10xe2x80x83xe2x80x83(7)
is applied to the conductive plate 500.
Next, it will be described how much the electric field decreases its intensity due to the existence of the leakage resistor 512.
The capacitance c511 (F) of the parallel plate capacitor 511 is given by
c511=xcex52xc2x7S/d2xe2x80x83xe2x80x83(8)
Supposing the resistance of the leakage resistor 512 is r512 (xcexa9), the electric field Ea(t) (V/m), which will be placed on the gate oxide film 501b when a period of time t (s) has passed, is given by
Ea(t)=E10 exp(xe2x88x92t/(c511xc2x7r512))xe2x80x83xe2x80x83(9)
FIG. 13 illustrates this decrease in electric field intensity with time. The period of time t5 (s), during which an electric field equal to or intenser than E1 (V/m) is placed on the gate oxide film 501b, is given by
t5=c511xc2x7r512xc3x971n(E10/E1)xe2x80x83xe2x80x83(10)
Accordingly, during this period of time t5 (s), an electric field stress with an intensity equal to or greater than the predetermined field intensity E1 (V/m) is continuously placed on the gate oxide film 501b. That is to say, the gate oxide film 501b is subjected to a burn-in test for this period of time t5.
In this method, however, the time t5 (s) is determined by only four process constants of c511, r512, E10 and E1. Accordingly, unless the process conditions are changed, the burn-in period cannot be extended.
In addition, no reverse electric field is applicable to the gate oxide film, so devices with early failures can be screened out far less completely. Furthermore, the current flows through the substrate always unidirectionally except the initial state. Accordingly, not so much stress can be placed on lattice defects that exist either in the substrate or around the interface between the gate electrode and the substrate. Thus, those failures can be screened out only insufficiently.
It is therefore an object of this invention to get the burn-in period changed by various parameters other than those process constants for a semiconductor wafer test system for use in a burn-in test on semiconductor devices.
Another object of this invention is to make a reverse electric field applicable to the devices under test.
Still another object of this invention is to place a sufficiently high voltage stress on lattice defects existing in the substrate or around the substrate/gate electrode interface.
To achieve these objects, according to the present invention, a semiconductor wafer under a burn-in test is exposed to either electromagnetic wave or alternating-current electric field.
Specifically, an inventive semiconductor wafer test system is a system for carrying out a burn-in test on a great number of semiconductor devices that have been formed on a semiconductor wafer. Each said device includes a gate oxide film between a substrate and a gate electrode. The gate electrode is connected to a metal interconnect. The system includes electromagnetic wave generating means. The generating means exposes the wafer to an electromagnetic wave as an alternating current wave and places an electric field with a predetermined intensity on the gate oxide film of each said device on the wafer, thereby carrying out the burn-in test on the devices.
Another inventive semiconductor wafer test system is a system for carrying out a burn-in test on a great number of semiconductor devices on a semiconductor wafer by exposing the wafer to an alternating-current electric field, not the electromagnetic wave.
In one embodiment of the present invention, the inventive system may include stress sensing means and control means. The stress sensing means senses a voltage stress imposed on the gate oxide film of each said device while the wafer is being exposed to the electromagnetic wave or the alternating-current electric field. The control means controls the intensity of the electromagnetic wave or the alternating-current electric field so that the voltage stress sensed by the stress sensing means falls within a preset threshold value range.
In this particular embodiment, the voltage stress, which has been sensed by the stress sensing means as being imposed on the gate oxide film, preferably includes forward and reverse voltage stresses. The control means preferably controls the intensity of the electromagnetic wave or the alternating-current electric field so that the forward and reverse voltage stresses imposed on the gate oxide film fall within first and second preset threshold value ranges, respectively. In this case, the second range is preferably lower than the first range.
Still another inventive semiconductor wafer test system is a system for carrying out a burn-in test on a great number of semiconductor devices formed on a semiconductor wafer. Each said device includes a gate oxide film between a substrate and a gate electrode. The gate electrode is connected to a metal interconnect. The system includes electric field generating means and driving means. The generating means includes a conductive plate for exposing the wafer to an electric field as a direct current wave. The generating means sets the electric field placed on the gate oxide film of each said device on the wafer to a predetermined intensity. The driving means loads and unloads the wafer into/from a space where the electric field, generated from the conductive plate, exists. In this manner, the wafer is exposed to an alternating-current electric field to carry out the burn-in test on the devices.
According to the present invention, a semiconductor wafer can be exposed to electromagnetic wave or alternating-current electric field for an interval of a variable length. Thus, a burn-in test can be carried out on semiconductor devices on the wafer for any arbitrary period of time. In addition, a reverse electric field is also applicable to the gate oxide film of each of those devices. Accordingly, devices with failures can be screened out with much more certainty. Also, a sufficient stress can be placed on lattice defects existing in the substrate or around the substrate/gate electrode interface.
Moreover, according to the present invention, a reverse voltage applied to the gate oxide film of any semiconductor device is set no greater than the maximum allowable reverse voltage of the gate oxide film. Thus, the semiconductor devices can be tested without deteriorating the gate oxide film of any normal one of the devices.
Furthermore, according to the present invention, the driving means alternately loads and unloads the wafer into/from a space where the electric field generated from the conductive plate exists. Accordingly, it is possible to expose the semiconductor devices on the wafer to an alternating-current electric field and freely set the burn-in period to any arbitrary length while using a direct current power supply.